Chemical reactions are the process of arranging atoms and bonds when chemical substances come into contact. Chemical properties change by altering the arrangement of the atoms in that substance.
The chemicals that change are the reactants and the new substances that are formed or appear are the products:
The chemical equations show what are the reactants, the products and other participants as catalysts and energy. The reactants are noted to the left of the arrow pointing to the products, in this way:
This is read as follows: a molecule of methane gas reacts with 2 molecules of oxygen to form a molecule of gaseous carbon dioxide and two molecules of water vapor.
Oxygen and methane are the reactants, carbon dioxide and water are products.
Characteristics of chemical reactions
- They can be reversible or irreversible: If the products become reactants, the reaction is said to be reversible. If, on the other hand, when the products do not re-form the reactants that gave them their origin, we talk about irreversible reactions. For example, when sugar is transformed into caramel when heated, no matter how much we cool the caramel, it is impossible for it to turn into sugar again.
- The atoms do not change: in a chemical reaction, the atoms remain the same, what changes is the way they are united between them.
- Most of the reactions that occur in organisms take place in aqueous solutions, that is, the substances are dissolved in the water.
- They can be simple, they require only one step for the reactants to become products. Other reactions are more complex, so they have several steps between reactants and products. Between the steps, intermediate compounds can be formed, which are neither reactants nor products.
- The amount of atoms in the reactants is equal to the number of atoms in the products.
Balance the chemical reaction equation
In a chemical reaction the number of atoms does not change : if there were 10 hydrogen atoms in the reactants, there must be 10 hydrogen atoms in the products.
When we check that between reactants and products the amount of atoms are equal, we are talking about balancing the reaction equation . For example:
Hydrogen and oxygen are diatomic molecules, that is,
But on one side there are 2 oxygen atoms and on the other side of the arrow there is a single atom of oxygen. We fix this by placing a 2 in front of the water:
Now we have two oxygen atoms on the left and two oxygen atoms on the right. We place a 2 in front of the H 2 on the left and the equation is balanced:
How do chemical reactions occur?
Chemical reactions occur due to the collision or collision between reactants.
For the reactions to take place, the reactants must collide. Energy is required to break the bonds between atoms and when new unions are formed, energy is released.
The theory of collision states that the collisions between molecules can provide the energy necessary to break the bonds between atoms and in turn generate the energy to form new unions
An exothermic reaction is one where thermal energy is released, that is, the reactants have more energy than the products. Exo = exterior, releases energy.
An endothermic reaction is one where thermal energy is absorbed, that is, the products have more energy than the reactants. Endo = inside, energy enters.
When we cook we are facing an endothermic reaction.
Types of chemical reactions
Depending on what happens between reactants and products, the chemical reactions can be the following:
Precipitation reactions
When sodium chloride is added to a silver nitrate solution, the water-insoluble silver chloride drops.
When two solutions are mixed and a solid substance is formed that does not dissolve, we are in the presence of a precipitation reaction. The solid that forms is called a precipitate .
Precipitation is the process by which when a salt is formed from its soluble ions, it falls because it is not soluble in water.
Example
When AgNO 3 aqueous silver nitrate is added to a solution of KCl potassium chloride, silver chloride AgCl is formed which precipitates as a white solid. The reaction is as follows:
We can know if a compound is soluble or not in water by following these rules:
| Soluble | Insoluble |
|---|---|
| All compounds containing ammonium NH 4 + or alkali metal cations | All carbonates, sulphites and phosphates (except those formed with NH 4 + ammonium and alkali metal cations) |
| All nitrates and acetates | All hydroxides (except those formed with NH 4+ ammonium and alkali metal cations) |
| All chlorides, bromides and iodides (except those formed with Ag, Pb, Hg) | All sulfides (except those formed with NH 4 +ammonium and alkali metal and alkaline earth metal cations) |
| All sulfates (except those formed with Ag, Pb, Hg (I), Ba, Sr, Ca | All oxides (except those that are formed with Ca, Ba, and alkali metal cations) |
Alkali metals: Lithium (Li), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), Francio (Fr); alkaline earth metals: Beryllium (Be), Magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium (Ra).
Acid-base or neutralization reactions
As the base is added to an acid, the pH increases.
An acid is a substance that increases the concentration of hydrogen ions (H + ) in a solution when some of its hydrogen atoms are dissociated. A base provides hydroxyl ions OH – or another negatively charged ion that reacts with H + in solution, lowering the H + concentration .
When an acid and a base react, a double displacement or neutralization reaction occurs because water forms. Example:
This reads: a molecule of sulfuric acid (H 2 SO 4 ) reacts with two molecules of sodium hydroxide (NaOH), forming a molecule of sodium sulfate (Na 2 SO 4 ) and two molecules of water.
Oxide-reduction reactions
In the reactions of oxide-reduction or also called reactions redox there is transfer of electrons between the reactantes. This transforms the oxidation states of the elements or compounds. The corrosion of iron tools is a classic example of an oxidation-reduction reaction.
Oxidation refers to the loss of electrons, reduction refers to the gain of electrons. An oxidizing agent is the species or substance that oxidizes another species, reducing in the process. A reducing agent is the species that reduces another species, oxidizing in the process.
In this case, hydrogen sulfide (H 2 S) is the reducing agent while chlorine (Cl 2 ) is the oxidizing agent.
Combination reactions
The combination reactions involve the union of elements or molecules to form a compound:
Example
Two sodium atoms combine with a gaseous chlorine molecule to give 2 molecules of sodium chloride.
Decomposition reactions
By means of electrolysis, water is separated into its components hydrogen and oxygen.
In the case of decomposition reactions, a compound is divided into its components:
Example
Two water molecules are divided into two molecules of hydrogen and one molecule of oxygen.
The zinc Zn displaces in the silver nitrate the silver Ag that forms species of trees.
In the displacement reactions, one of the reactants substitutes a part of the other reactant, as in the following case:
Reactant C displaced component A of the AB molecule.
Example
A gaseous chlorine molecule displaces bromine in two molecules of sodium bromide, producing 2 molecules of sodium chloride and one molecule of bromine.
Double displacement reactions
Copper sulfate reacts with sodium carbonate to give sodium sulfate and copper carbonate in a double displacement reaction.
Within the displacement reactions, we can get reactions where the parts of the reactants are exchanged. These are known as double displacement reaction:
This is read as iron oxide (III) reacts with six molecules of hydrochloric acid and forms two molecules of iron (III) chloride plus three water molecules.
Combustion reactions
A carbon-containing compound reacts with oxygen to produce carbon dioxide, water and heat.
In this case a propane molecule reacts with five oxygen molecules from the air and produces 3 molecules of carbon dioxide and 4 molecules of water. The most important product of this reaction is heat.
Dismutation reactions
In the dismutation reactions a molecule is reduced and oxidized at the same time. These reactions are rare. The classic example is the dismutation of hydrogen peroxide:
In this reaction the oxygen of H 2 O 2 has an oxidation state equal to -1. In H 2 O, the oxidation state is -2, meaning that it gained an electron and was reduced. In O 2 , the oxidation state is 0, that is, it lost an electron and oxidized.
Examples of chemical reactions
In our daily life we are surrounded by chemical reactions. Some of the most common examples below.
Reaction of vinegar and sodium bicarbonate
The reaction between vinegar and bicarbonate is the most used.
Acetic acid, which we commonly know as vinegar, is a weak acid. Sodium bicarbonate, which is a base, reacts with vinegar in a two-stage reaction. The first stage is a double displacement reaction:
Na sodium from the bicarbonate is changed to a hydrogen from the vinegar to form sodium acetate and carbonic acid. The carbonic acid is unstable and undergoes a decomposition reaction:
Water and carbon dioxide are produced in this reaction. The bubbles we see in this reaction is carbon dioxide in gaseous form.
Reaction of antacids
The hydrochloric acid (red) reacts with some antacid tablets and the solution is neutralized (green).
Hydrochloric acid found in the stomach can sometimes cause discomfort or ulcers. To neutralize it, there are different compounds that are used as “antacids” that are actually bases. Examples of the neutralization reactions of HCl with various “antacids” follow below:
Aluminum hydroxide plus hydrochloric acid forms aluminum chloride and water.
Calcium carbonate plus hydrochloric acid forms calcium chloride, water and carbon dioxide.
Magnesium carbonate plus hydrochloric acid forms magnesium chloride, water and carbon dioxide.
Magnesium hydroxide plus chlorine forms magnesium chloride and water.
The basic structure of a battery is made up of plates of lead oxide PbO 2 and lead Pb, bathed in aqueous sulfuric acid H 2 SO 4 . The reaction that occurs in the battery is as follows:
Lead acts as a reducing agent while lead oxide acts as an oxidizing agent.
Acceleration of chemical reactions
A chemical reaction can be made faster in several ways:
- Increasing the temperature: the speed of the reactants increases and they are more likely to collide with each other.
- Increasing the concentration of the reactants: as there is a higher concentration of the reactants, the probability of a reaction is greater.
- Using catalysts: these are substances that accelerate chemical reactions, without being part of the reactants or products. Living organisms depend on these types of substances, better known as enzymes, for chemical reactions to take place in cells.
Problems of chemical reactions solved
1. Based on the rules of solubility, predict which of the following substances are soluble in water:
Aluminum nitrate
Magnesium chloride
Rubidium sulfate
Nickel hydroxide (II)
Lead sulphide
Magnesium hydroxide
Iron phosphate (III)
2. When the following substances are mixed, what precipitate will be formed?
one) 
two) 
3) 
4) 
3. Write the balanced formula for the following acid-base reaction:
4. Write the balanced formula for the following acid-base reaction:
5. Write the balanced formula for the following acid-base reaction:
6. Some of the substances most commonly used as antacids are magnesium oxide MgO, magnesium hydroxide Mg (OH) 2 and aluminum hydroxide AL (OH) 3 .
Write the balanced equation for the neutralization of hydrochloric acid for each of these substances.
